Need to focus for entrance examination , Supplement recommendation ?

Soo hey guys i am currently studying for an entrance test and I am having difficulty in concentration so my test is working 50 days and o want some recommendation on something which can boost my brain activity ( don’t want drug’s I don’t want side effects ) can be any chewing gums or tablets which can just help me stay focused for few hrs so that i can study in these last few days o don’t wanna mess up my hard work but i just cant seem to study as the exam is approaching nearby….pls help your brother 😃😃